Method and apparatus for material analysis

ABSTRACT

Method and apparatus for material analysis in which X-rays generated pursuant to incidence of an electron beam on the material are detected by a detector which generates signals representative of X-ray intensity. A first single analyzer is connected to receive the signals from the detector and to pass to an associated first counter a count signal whenever the signal applied to the first single channel analyzer is representative of an X-ray energy within a relatively narrow range of such energies. A second single channel analyzer is also connected to receive the signals from the detector and to pass to an associated second counter a count signal whenever the signal applied to the second analyzer is representative of an X-ray energy falling within a much broader range of such energies than the first mentioned range. The first and second counters accumulate the count signals applied thereto. The count in the second counter is compared by a comparator with a pre-established count in a third counter and when the count in the second counter assumes the same value as the count in the third counter the counts in the first and second counters are held. The so held count in the first counter then itself represents a normalized ratio of X-ray energy within the narrow range to the X-ray energy for the energy spectrum represented by the broad range of energies. On the basis of this normalized ratio information as to the makeup of the material can be derived.

This invention relates to a method and apparatus for material analysisof the kind in which a beam of energy is caused to fall on a spot on thesurface of the sample of material to be analysed and signals, such asX-ray signals or backscattered electron signals are generated eachrepresenting an effect of the beam at the spot and from which signalsinformation as to the makeup of the material can be derived.

Methods of the above kind are adapted for use where a scanning electronmicroscope provides the beam of energy and it is principally in thiscontext that the invention is particularly described thereinafter.However, the invention is not confined to use of scanning electronmicroscopes.

The invention broadly contemplates, in one aspect, a method of analysisin which a beam of energy is caused to fall on a spot on the surface ofa sample to be analysed and X-rays then generated at the spot aredetected by one or more detectors to produce first signalsrepresentative of the energies of detected X-rays; a first count of thenunber of said first signals each representative either of an energywithin a relatively broad range of such energies or of a combination ofdiscrete energy intervals across said first relatively broad range saidcombination including coincident, anti-coincident or contiguousintervals being made and a second count of the number of said firstsignals each representative of one particular energy or of an energy inan associated relatively narrow range of energies about said particularenergy being made, wherein information relating to the relativeproportion of a particular chemical element, characterized by productionof X-rays of or about of said particular energy is obtained in the formof a normalized ratio of said second count to said first count, saidnormalized ratio being represented by the value of said second countassumed when the said first count reaches a predetermined value. Thefirst said count may as a particular case consist of the sum of all orsome subset of the second said counts.

In another aspect, the invention broadly contemplates apparatus formaterial analysis including:

energy generating and directing means for causing a beam of energy tofall on a spot on the surface of a sample of the material to beanalysed;

detector means for detecting X-rays generated at said spot and forproducing first signals representative of the energies of detectedX-rays;

first accumulating means coupled to accumulate a first count of thenumber of said first signals each representative either of an energywithin a first relatively broad range of such energies or of acombination of discrete energy intervals across said first relativelybroad range, said combination including coincident, anti-coincident orcontiguous intervals;

second accumulating means for accumulating a second count of the numberof said first signals each representative of one particular energy or ofan energy in an associated relatively narrow range of energies aboutsaid particular energy;

presettable means coupled to said first accumulating means andresponsive, on said first count reaching a predetermined value, tocontrol said second accumulating means to hold the value of said secondcount then assumed, whereby said assumed value represents a normalizedratio of said second count to said first count which normalized ratio isdependent on the proportion of a particular chemical element in saidsample.

In addition to the normalized value already notionally divided, anactual division can be carried out if desired and the results obtainedtherefrom may be further manipulated.

The invention is further described by way of example with reference tothe accompanying drawings in which:

FIG. 1 is a diagram illustrating the use of a scanning electronmicroscope to obtain image information from a sample;

FIG. 2 is a circuit diagram of apparatus useful in analysing X-raysignal information;

FIG. 3 is a circuit diagram of apparatus useful in processingbackscattered electron information;

FIG. 4 is a circuit diagram of a system for forming data words or masksfrom backscattered electron and X-ray count information;

FIG. 5 is a circuit diagram of a modification of the system of FIG. 4adapted for inhibition of storage of full word or mask data inaccordance with grey levels of backscattered electrons;

FIG. 6 is a diagram of a system for determining ratios of X-ray signals;and

FIG. 7 is a partly diagrammatic side view of a backscattered electrondetector array useful in the invention.

Generally, the aspects of the invention to be particularly describedrelate to an image acquisition and analysis system which, from amaterial sample, forms or allows to be formed, mineral, compositional,elemental or phase maps, or identifies at specified image points thephase, mineral composition, element or elements present at those points.The compositional map identifies in two dimensions the regions ofdifferent mineral or material composition. This map is formed in such away that it may be readily stored in a minicomputer or in the bulkstorage peripherals with which such a computer is fitted. The mode ofmap storage allows the ready treatment of the data to extract a largenumber of quantitative parameters describing the objects viewed in thescanning microscope field. These objects may be individual particles,and the particles may themselves contain a number of different species.Alternatively the sample may be a section of an ore or other materialoccupying the whole or part of the scanned field. The image acquisitionsystem uses severally and in combination elemental X-rays and thesecondary, backscattered or absorbed electron signals created forexample when an electron beam, a collimated X-ray fluorescence beam, anion beam or a proton beam falls on a defined spot on a sample. The setof such spots is defined as the image raster, and in general the beam ofcharged particles or protons which is creating X-ray, electron, orproton signals is moved or positioned in a regular pattern. The presentinvention allows elemental X-rays and backscatter electron signalintensities generated by the beam of a scanning electron microscope tobe used to define the mineral, composition, phase or element present ateach point. These signals may be processed preferably in real time toproduce in a rapid manner a digital word or mask which defines, orserves to define, the mineral, composition, phase, or element at eachpoint of the raster and to allow the formation of a map or image ofthese species separately or in combination, and the transmission to acomputer or similar memory a condensed version of the data defining sucha map or image, all of these procedures being accomplished during theactual scan of the sample or a defined region of it.

The sample may consist of a polished or moderately rough specimen orsurface of an ore or rock, a metallurgical or ceramic material, or anysimilar or comparable material. Particles or fragments of such materialmay be cast in epoxy, plastic, or other media and the sample thenconsists of a section exposed by cutting through the casting. Inparticular, the sample may also consist of individual fragments orparticles, placed on or adhering to a supporting surface, and acrosswhich the irradiating beam is moved or onto each of which it isdirected.

The form of the invention to be described permits the formation ofimages in which the spatial location and correspondences betweenmineral, compositional, phase or elemental species are determined. Themethod may be used for initial location followed by quantitativeanalysis of specific minerals and phases, for example individualmonophase particles or objects in a field of view, or for exampleregions consisting of a given mineral in particles or sections. Once thedescribed interpretive procedure has located a specific or requiredphase, the electron beam may be redirected to it, and X-ray acquisitiontimes long enough to allow quantitative analysis can be used. Forexample the iron content of sphalerite, (Zn,Fe)S, can be determined inthis way after the species has been initially located and identified bythe methods of the invention.

From compositional or species maps or images the procedures to bedescribed can abstract stereological, spatial, textural or metricparameters or information regarding the objects or adjacent areas in theimage or sample. The procedures also include modal analysis, asgenerally defined in quantitative minerology or metallography and theanalysis of individual particles either in terms of their mappedcompositional features or as monophase individuals.

The apparatus and methods to be described provide for the rapid realtime accumulation of an image map, or of image lines or points, withinwhich compositional species such as minerals, metallurgical phases, orindividual elements are identified. The map, line or point data can beproduced in a condensed form which minimizes data transfer from theimage acquisition system to a memory device, such as the memory of astandard minicomputer, and provides an image data format which can bedirectly manipulated by algorithms which take maximal advantage of thiscondensed format. Compositional bit patterns for every point in a rastercan also be transmitted or stored when required.

In the following description real time is defined to mean an event,decision or procedure carried out during the course of an operation, inparticular during the process of positioning the irradiating beam at asequence of spatially defined points so as to create a digital imageraster or part thereof. Backscattered electron(s) is abbreviated to BSE,energy dispersive X-ray analyser or analysis to EDS. The embodiments ofthe invention to be described facilitate the formation of a map of amineral section or of a large number of particles to be formed within apractical time, fractions of seconds for BSE signals alone, or minutesto tens of minutes for X-ray signals, and the subsequent treatment ofthis data in times of the order of seconds or minutes in order todescribe quantitatively the properties of populations of compositeparticles, or of ore sections and of solid material samples generally.

Accumulation of the whole or part, or more than one part, of an EDSspectrum during a period just sufficient to provide a "yes" or "no"response to the presence or absence of a given element or group or setof elements may be effected, and the selection from within the variousspectral regions of those groups of responses, e.g. successive memory orevent counter locations, which serve to characterize each identifiableelemental X-ray, and the performance of an examination or decision onthe contents of such groups or on the combinations of their contents, soas to provide a digital word or mask which characterizes the species orcomposition of the raster point from which the X-ray and BSE data isderived. Such a word or mask is or can be formed in real time either inshort intervals between the sampling of X-rays and/or BSE signals ateach raster point, or is formed for a given raster point during the timeinterval during which the data from the next raster point is beingcollected. Alternatively a series of mask values can be saved until atime interval sufficient for interpretation occurs.

One or more wavelength spectrometers may be used in combination with EDSand formation of a raster or line image by movement of the electronbeam. Automatic movement of the sample so that the electron beam can beswept over a new area or line may be practised when practising thepresent invention.

In FIG. 1 a conventional scanning electron microscope (SEM) 10 is shown,this having a source 12 in use producing a collimated beam 14 ofelectrons which is directed onto a sample 16 to be analysed. The SEM hasdeflecting means 18 operable to cause the beam to scan the surface ofthe sample in a suitable raster pattern such as the series of parallellines 20 shown. The beam is moved along each line in succession andcaused to pause at successive ones of the series of spots 22 in eachline. X-rays and backscattered electrons produced at each of the spotspass two detectors 24, 26 respectively from which are producedrespective first or X-ray signals and second or BSE signals onrespective lines 28, 30. The X-ray or "EDS" detector 24 is an energydispersive detector of conventional form and produces, for each spot, atime spaced series of X-ray signals of amplitudes which arerepresentative of energies of X-rays generated at that spot pursuant toincidence of the beam thereof. BSE detector 26 may likewise be ofconventional form, producing the aforementioned BSE signal therefrom asan analogue signal representative of the intensity of backscatteredelectrons at the spot upon which the beam 14 is incident. By examiningthe signals from detectors 24, 26 it is possible to deduce propertiessuch as elemental or phase compositions of the sample 16. Moreparticularly, Table 1 shows examples of numerous mineral phases whichare recognizable by examination of peaks in X-ray energy produced byincidence of beam 14 on a spot 22. These peaks are represented by maximain the numbers or "counts" of the aforementioned X-ray signals from theX-ray detector which are of particular amplitudes. The invention makesuse of these counts and "spectrum" counts in a particular way tofacilitate analysis of the sample. This is accomplished by using thewhole or a sufficient part of the X-ray energy spectrum, andparticularly by the use of two or more regions of the spectrum, orcombinations of such regions as count values against which to normalizeX-ray peak or peaks of a given element(s) in a given phase.Alternatively the number of counts in the peaks may be summed, with orwithout background subtracted, to obtain a similar useful result. In oneversion, X-ray count accumulation at a given electron beam dwell pointis continued only until the counts for the total spectrum, or asufficient part of it, or for two or more parts independently, equalpreset values. This value, or values, which will usually also includeall other X-ray counts due to given elements present in the sample isalways higher than the value for a given peak, and is in many cases muchhigher. Thus normalization always occurs with a statistical accuracyhigher than, and often much higher than, that obtained from any givenelemental X-ray peak. The time to accumulate the counts for such a peak,with a given expected confidence level, is thus determined by theaccumulation of the total spectrum, or a sufficient part of it, and notmerely by accumulation of the peak itself, and essentially the sameconfidence level is achieved whether the peak is there or not.

The counts corresponding to a given element in a given phase will thenhave a predetermined value (with an estimated standard deviation of thesquare root of the value) if the electron beam spot is incident on sucha given phase or composition. If the beam is incident on some otherphase, which does not contain the given element, only background counts,to a specified value, will accumulate in the given channel. The countvalue in the channel corresponding to the specified element isindependent of the time taken to accumulate the spectrum. In real timeimage formation this means that the beam is only kept at a given imagepoint just long enough to accumulate sufficient spectral counts todefine a desired confidence level in the various peaks to be defined inany one of them, and an arbitrary dwell time at each image point neednot be used during which either more or fewer counts than are needed areaccumulated. For a rough or sloping surface, from which counts areobtained more slowly due to sample-detector aspect or to increasedabsorption of X-rays within the sample material, automatic accommodationis made, and for multiple detectors, shading of one or more does notaffect the outcome. In addition, the background corresponding to theenergy window width within which a given X-ray spectral peak from agiven phase or composition is sampled, is also normalized with respectto the total spectrum or part thereof, and can thus be preciselydefined.

It has been found that independent accumulation of two regions of theX-ray energy spectrum, namely 0.80 to 4.20 KeV, and 4.20 to 20.0 KeV,allowed reproducible X-ray count values, well within the limits requiredfor real time elemental identification by thresholding or by takingratios, to be obtained from polished sample surfaces lying at anglesbetween 60° tilted towards a detector located at 35° to the horizontaland at up to 35° tilted away from the detector. Other convenient energyranges can also be used.

The time taken to accumulate the separate regions of the spectrum canvary widely with angle as shown in Table 2, but the counts in a givenelemental X-ray channel normalized to the spectral region counts, werenone-the-less highly reproducible as can be seen from Table 2. Even forwhole, three dimensional, particles with fractured or rough surfaces,where various scattering and absorption effects reduce the X-ray countsin a given peak with respect to spectral or local background, the ratiosbetween X-ray peak counts for pairs of elements in a given phase remainsubstantially constant.

The cumulation rates in two or more regions of the spectrum may beseparately monitored. If such monitoring shows that the requiredspectral count will not be attained in a specified time, counting forthat region of the spectrum, or for the whole spectrum, is discontinuedand the beam is moved to the next raster point. This limits the timespent at any one point if no or insufficient X-rays are being produced,or if significant rates of elemental or background counts are producedin one of the normalized regions only. Since the X-ray background countvalue in a region without peaks is dependent on the average atomicnumber of the material under the beam producing peaks in other spectralregions and the backscattered electrons signal level is also a functionof this average atomic number the BSE signal level can be used todynamically modify the threshold values at which counting isdiscontinued.

The value of 4.20 KeV or some value, experimentally adjustable, in theclose vicinity, optimizes for example the sulphur/iron count ratiodiscrimination between compounds such as FeS and FeS₂. In general, asshown by the examples in Table 2, use of the specified regions enablesconsistent ratios to be obtained between X-ray peaks in the region 0.8to 4.2 KeV and those in the region 4.2 to 20 KeV. Values close to 4.20KeV also have the advantage of being in the region where scandium Kα andKβ X-rays occur. As this element is rare, there are usually no peaks inthis region.

One means of accomplishing the above consists of presettable digitalcounters following one or more single channel energy windows whichaccept all or a specified part of the total spectrum and which can bepreset to a given value. The signal resulting from this counter when thepreset value is reached is used to shut off the accumulation of countsin the various channels receiving X-ray counts corresponding to a givenelement or group of elements, or to the background in the window regionof such a peak, or for any other part of the spectrum which is ofinterest. This signal, or a sequence of such signals, also triggers thecomparison circuits of the digital scan control system, which thenproceeds as at the end of any usual or arbitrary beam dwell period.

Notwithstanding the use of peak to spectrum ratio normalization asabove-described, it is also possible to use an anti-coincidence peak tospectrum ratio method in which the spectrum or relevant portion of thespectrum does not include the peak or peaks in question. This isachieved by taking the counts accepted for a given peak or set of peaks,cumulating them in each of the counters appropriate to the given elementor set of elements, and at the same time, or with a short delay toaccommodate the circuit logic, subtract them from the total cumulatedspectrum or part thereof, either in the spectrum normalization counter,as above, or in a separate counter. Alternatively, the counts within apeak or peaks can be continually added to the preset value against whichthe total cumulated spectrum or part thereof is compared. Thus thenormalizing spectrum can be made to consist either of total counts in aspectral region, or of those counts in a region not including the peakor peaks of interest, or alternatively of the total counts of all peaksof interest excluding the regions not in the peaks, or of some prechosencombination of regions. This normalization is particularly useful indiscriminating different minerals with common elements on the basis ofthe amount of that element present. For example the ratio of a givenpeak to the remainder of the spectral region can be taken and this ratiocan be again taken as a ratio against the ratio of another peak to theremainder of spectral region in an alternate, or the same part, of thespectrum. For the examples given in Table 2, discrimination of FeS₂ andFeS on this basis would give results shown in Table 3.

There is thus produced a 50% differential between the A/B ratios for FeSand FeS₂, as compared with a 17% differential for direct S/Fe ratios,and discrimination is accordingly considerably easier.

A suitable means of implementing the invention is shown in FIG. 2. InFIG. 2 single channel analysers (SCAs) 1-N and P-Z, or their digitalequivalents, each have their windows set to allow passage of signalsfrom X-ray detector output line 28 representative of a particularspectral peak, expected when a particular element is irradiated by theSEM electron beam.

Two separate SCAs, BG-1 and BG-2, have their pass windows set muchwider. BG-1 accepts pulses, for example in the 0.8 to 4.2 KeV range, andBG-2 in the 4.2 to 20 KeV range. Through these SCAs the total countsoccurring in each of the spectral regions are accumulated in countersB-1 and B-2, where they are compared with those in presettable countersA-1 and A-2 by means of comparators C-1 and C-2. In the simplesttwo-region mode, A-1 and A-2 are preset with given values e.g. 330 and500. When B-1 has collected 331 counts its comparator sets high andindividual counters 1, 2 . . . N are prevented from accepting furthercounts via signals on `Hold` lines 32. Similarly, and independently ofB-1, B-2 collects 501 counts before counters P, Q, . . . Z are held attheir current contents value. Response of both comparators signalscompletion of the sequence. If neither B-1 nor B-2 reach the presetvalue in a pedetermined time, typically 40 or 50 milliseconds, a timerT-1 halts all individual counters.

Termination of the X-ray count collection period by either method causesa signal to appear at "Accept Condition Code", this signal being,generated by a monostable 34 responsive through gates 36, 38 to theconditions of comparators C-1, C-2 and of timer T-1. Appearance of the"Accept Condition Code" signal initiates comparison of the contents ofcounters 1, 2 . . . N and of P, Q . . . Z with preset values or may beused in ratio calculations as described later. All counters are thencleared to zero, A-1 and A-2 are preset to 330 and 500, the beam movesto the next spot in the raster and counting is commenced at this point.These operations are effected after delays following appearance of the"Accept Condition Code" signal, the delays being induced by delaycircuits 42, 44. If it is desired to compare a given peak or peaks withthe remainder of the spectrum in the given spectral region as, forexample, counts in the FeKα peak with the remainder of the spectralcounts in the 4.2 to 20 KeV region, counter A-2 can be set to 500 priorto counting and, as counts occur in the desired peak, say throughSCA-`P`, they are added continuously to those in A-2 thus maintainingtheir difference. The counts in this peak have thus been normalized tothose in the spectral region less the peak counts themselves. The peakcount value, notionally, corresponds in this case to the ratio P/(S-P)where P stands for the peak counts and S for the total spectral regioncounts including the desired peak. As in the simpler case, P/S, it isnot actually necessary to perform this computation in order to determineif the ratio would fall into a given value range, but merely to inspectthe value P contained in counter P. Similarly with counter 1 collecting,331 counts are preset the 0.8-4.2 KeV region is used to normalize thepeaks occurring in this region. By appropriate adjustment of theacceptance ranges for ratio calculations, as described later, anormalized iron to sulphur X-ray count ratio can be obtained by takingthe ratio in counts from CTR.P to those from CTR.1 with final results asgiven in Table 4.

More than one peak at a time can be added to counters A-1 or A-2 by theuse of switches, or gates, on the inputs to OR-gates 40 connectingbetween SCA-1 . . . N and A-1 and SCA-P . . . -Z and A-2.

In some cases it is sufficient for normalization of counts within peaks,optimization of counting time, and attainment of given confidence levelsto feed all counts from 0.8 to 20 KeV into one counter, either B-1 orB-2 or to preset both counters to the same value, in order to normalizethe counts within all peaks examined (CTR.1 . . . N and CTR.P . . . Z)to the total spectrum, or to the total spectrum less the counts in thepeaks.

The above methods may be further refined by prior estimation of the peakto background ratio for the actual channel or channel group accepting agiven peak, and including a logic divider or other device which reduces,in the ratio of background to peak, the number of counts beingsubtracted (i.e. those in anti-coincidence) from the spectrum. In thisway only the elemental X-ray counts and not the background counts at thesame energy are subtracted from the spectrum.

In summary, the above particularly described method provides a methodfor the normalization of X-ray counts whereby:

(i) a predeterminable background for a peak can be set and obtained;

(ii) a predetermined number of counts can be obtained in the peak(subject to normal random X-ray statistics, i.e. a count of N with astandard deviation of ±√N);

(iii) the time taken to accumulate an image based in whole or part onelemental X-ray counts or combinations of them, can be optimized whileat the same time a desired confidence level for compositionalidentification at each point of the image raster can be achieved;

(iv) the ratios between given peaks in an EDS spectrum of a given phase,or discriminant ratios between X-ray peaks for different phases can bedetermined and set with a definable confidence level;

(v) all discriminations based on windowing or thresholding or on ratiobetween different X-ray peaks counts, and all such X-ray identificationprocedures including full spectral peak stripping by separate software,can be made independent of electron beam current values or drifts,independent of the rate of accumulation of X-ray counts, and independentof the efficiency of a given EDS counter or of the use of more than onesuch counter even when variations in surface slope prevent one or moreof a set of multiple detectors from receiving X-rays;

(vi) a peak to spectrum ratio can be obtained in which the spectrum orspectral region does not necessarily include the peak counts themselvesfor the element or elements of interest;

(vii) the above properties can be obtained even on strongly slopingsurfaces or on the surfaces of particles or fragments, substantiallyindependent of the slope or roughness of such surfaces, and sufficientlyindependent to allow element or phase identification of materialsexposed in such surfaces;

(viii) the above properties can be obtained essentially independently ofthe nominal take-off angle or angles of the X-ray EDS detector ordetectors.

The BSE signals produced from BSE detector 26 are also useful inanalysis of sample 16. These signals can be used for example to build upa "photograph" like image of the surface of the sample. For analysispurposes in accordance with this invention, the BSE signals arepreferably sampled at each raster spot and then digitized. Moreparticularly, the signal at each spot is preferably allowed to rise toits full value before being sampled, and then integrated for a specifictime period and this integrated value taken as the measure of the BSEsignal, this measure then being converted to a precise digital value,e.g. with a resolution as great as that of the signal to noise ratio,its occurrence within a predetermined value band can be used to generatea specific digital response indicating an occurrence within that band.Value bands can extend across any part of the total range, and may becontiguous or overlapping. 8 such bands may be used, with a digitalresolution of 1 in 16. A resolution of 1 in 32 or 1 in 64 could equallywell be used depending on signal to noise ratio in the analogue signal,which in the present example is 50:1 at mid range (backscatter electroncoefficient of 0.3) using a solid state silicon semiconductor detector.Other detectors such as optical scintillator types could also be used. Aschematic diagram of the BSE signal processing unit is given in FIG. 3.In FIG. 3 the processed analogue signal, derived from the BSE intensity,is passed through a signal processing circuit 50 and thence passed to anintegrator 52 where it is integrated continuously and locked in by asample/hold circuit 54 at the level attained a preadjusted time afterthe beam has stopped at its current dwell spot. An analogue/digitalconverter 56 is connected to S/H circuit 54 and effects analogue/digitalconversion some very short time after lock-in and an End of Conversionpulse becomes available when A/D conversion is complete. Operation ofthe S/H circuit 54 and the converter 56 is effected under control of anadjustable timing circuit 60. The digital value obtained from converter56 is then true and one of 16 corresponding possible grey levels is setin a decoder 58. If this grey level falls between set limits slicecomparators 62 connected to decoder 58 will have corresponding outputbits set. Eight slice comparators are shown, combining to define an8-bit pattern. This current pattern is available to latch `A`. At theend of A/D conversion the contents of latch `A` are transferred to latch`B`. A fixed delay time later set by delay 64, the new current bitpattern is latched into latch `A`. Comparator `C` continuously checkslatch A against B and if they are different changes its output to a highstate. A further delay time later set by delay 66, a monostable 68 isactivated producing an extended pulse if latch A is different from B, nopulse if they are the same.

Use of this BSE signal processing in a system which identifies ordiscriminates phases by their backscatter electron response, avoids asuccession of spurious identifications as the signal value rises orfalls to a new final value when a new phase is encountered. Also, thedegree of confidence of identification is increased by converting anintegrated signal rather than an instantaneous or dynamically sampledvalue. Furthermore, the quality of the visual or photographic imageproduced by real time display of the sampled and processed image pointsis more uniform and better discriminated in region of the same ordifferent composition respectively, and the procedure acts to enhancethe visual or photographic image obtained from an SEM.

BSE and/or X-ray count information may be used to construct in real timeand at high speed a digital word or mask corresponding to a givencomposition or phase at a given image point or spot, the said digitalword containing values 0 or 1 at defined positions to define thepresence or absence of a given element on the basis of X-ray eventsand/or a digital value or bit pattern representing the BSE brightnesslevel. The above-described real time identification of mineral or otherphases is by use of BSE brightness levels alone or in combination withX-ray identification. When the latter are not required for any or everycompositional identification, the correspondence between each of thechosen BSE value bands to a given mineral or phase may be established byprior EDS identification of the various phases of interest. An exampleof the simultaneous quantitative evaluation of the phases present in anore section by means of BSE signals alone and by means of X-ray plus BSEsignals is given in Table 5.

The digital word is formed by the combination of digital "yes", "no"responses to a series of events or the encoding or acceptance of digitalvalues from the various analogue or numerical responses available in thesystem when the irradiating beam is caused to dwell at a given rasterpoint. In particular, the electronic pulses corresponding to a givenenergy range for a given elemental X-ray detected by an EDS analyser canbe fed to a set of parallel analogue discriminators (single channelanalysers) each followed by a counter whose contents are compared with apreset value or the occurrence of a digitized value of the pulse valuefalling into a given range can cause a count pulse to be fed to such acounter. If the preset value is reached during a given dwell or countperiod, or during a controlled count period as described below, thepresence of the element or of a given concentration range of theelement, or of more than one concentration range, is accepted and adigital bit is set at 1 in a defined position in the digital buffer. Ifthe element is absent the bit is set at zero. Such 1, 0 digital valuesmay also be derived from X-ray ratios as later described. Use is made inthe preset counters or comparison values, of values which justdiscriminate, at a 99.5% or other convenient confidence level, X-raypeak counts from background counts. A raster display screen may beprovided on which the yes/no responses from each or any of the X-raychannels is displayed as a bright dot as the scan proceeds, thuspermitting an operator to adjust the preset count level until visualinspection shows him that the response obtained for a given preset countvalue corresponds to a specific image area known to represent theelement or mineral or phase sought.

The digital buffer holding the digital word defining the speciescomposition is also caused to accept a digitally coded value whichspecifically designates the value band into which the BSE response ofthe species falls, as described above, with results typified by those inTable 1.

An example of the implementation of the method of the invention, undercontrol of a computer (not shown) is shown in FIG. 4. In FIG. 4, X-raypulses from the EDS detector are level discriminated in the set of SCAs.SCA-1 . . . SCA-N and SCA-P . . . SCA-Z previously described. Slicers70¹, 70², . . . 70^(n) (left of page) accept the SCA outputs and comparethe number accumulated in an alloted dwell time with upper and lowerlimits. If the number lies within these limits an output bit is set byclosing an associated switch 72¹, 72² . . . 72^(n) if not the output bitis reset. The accumulated count in certain counters is compared byslicers 74¹, 74² . . . 74^(m) with the value accumulated in others, toproduce a ratio of counts between given pairs. This ratio is comparedwith upper and lower limits and an output bit is set by closure of anassociated switch 76¹, 76² . . . 76^(m) if within limits, reset if not.Backscattered electron signal amplitude, a function of the averageatomic number of the mineral under the beam, is sliced into 16 greylevels by slicers 78¹, 78² . . . 78^(p). Eight upper and lower limitsare set by closure of respective switches 80¹, 80² . . . 80^(p) and ifthe signal amplitude falls within a defined range an output bit it setif not it is reset.

Before the beam is allowed to proceed to the next spot in the scanraster, the current bit-pattern, derived from X-ray and BSE bits, andrepresented by `new data` latch 86 is compared by a comparator 90 withthe pattern at the previous spot represented by previous data latch 88in `Compare A-B`. A detected change produces an active status conditionset in a latch 92. When this condition bit is set the computer respondsby reading the X and Y co-ordinates of the raster spot and currentpattern, after which the beam scan is restarted.

By switching off all but one X-ray count or ratio slicer output thepresence or absence of a single element or mineral may be monitored on abrightness modulated screen, responding to the 16-bit OR-gate, C1 andbrightness modulator 94. The screen may optionally be used to displaypoints of change of homogeneous run lengths. Grey level slicers may bemonitored on a second screen through gate D1.

If elemental identification by X-rays is not required at all points,such identification, with its longer dwell time, may be limited to areaswith a particular grey tone. The dual scan generator system may beswitched from X-ray speed and step-interval to BSE speed and interval byentering one or more grey slicer outputs into OR-gate E1, the output ofwhich selects the scan, by gating either a BSE scan generator 96 or anX-ray scan generator 98 through a gate 100 to a control 102 for thebeam. In this regard, collection of X-ray data requires longer timeintervals than BSE data, so that faster scanning is employed when onlyBSE data needs to be recorded.

If some changes in grey-tone are not required as computer data the greylevel slicer output bits may be batched in AND-gate F1. This forces theactive `Status` to inactive on selected grey levels.

The input to Multi-channel Analyser G1 may be opened on selected batchesof grey tones to provide a cumulative spectrum collected on areas ofinterest. The spectrum may be transferred for analysis and/orverification at the end of each scanned frame or be allowed toaccumulate over many frames for an overall spectrum of the whole sample.

A major advantage of the method used is that a minimum dwell time can beselected which minimizes image acquisition time for a given real timereliability in the recognition of given sets of elements andcombinations of elements in the sample or specimen.

In the present example the elemental X-ray responses or their ratios arecontained in up to 24 bits and the digitized electron or photonresponses in a further 8 bits. Other bit combinations can obviously beused.

The comparator 90 permits the digital mask formed at each raster pointto be compared with that at the previous point. When these values arethe same the scan generator may be arranged to automatically continuewithout communicating with the computer or memory. When they aredifferent the scan may be halted, the system computer called by a signalfrom the scan generator and the X and Y co-ordinates and digital mask ofthe dwell point transferred to the computer. This technique obviatestransfer of redundant image points, thus leaving the computer free forother tasks in the transfer intervals, and in particular allows theimmediate storage of the image in condensed run-length form in which asuccession of image points having the same composition are defined bytheir first and last positional values only.

The two independently and automatically controllable dwell timesprovided by generators 96, 98 may typically be of 1 to 1000 microsecondsfor BSE sampling and of 1 to 1000 milliseconds for X-ray sampling. Avery rapid movement of the beam from a given point to the next spatiallydefined point in the digital raster is also provided, independent of thedwell time or times employed at the point or points. The sweep speedbetween dwell points is derived from a 10 MHZ or 20 MHZ digital pulsegenerator driving a digital to analogue converter.

The spatial co-ordinate values and compositional mask of a particularraster spot may be arranged to be not recorded or transferred to anydesired memory array until a difference between successive masks isfound.

The invention can be adapted for minimizing the amount of data storedand manipulated during image analysis of particles or areas of interest,both simple and complex, within the sample field.

Thus, there may be formed, during raster scanning at high speed, alimited binary or multinary image based usually on BSE signals alone oron BSE signals plus limited X-ray signals, from which the minimuminformation is extracted necessary to define the minimum and maximum Xand Y co-ordinate points of each object or area of interest.

The list of such values is stored, and the scan generator is thenaddressed with the set or values for one object at a time. The limitedarea, usually rectangular, enclosing this object is then sampled orscanned in detail using the various aspects of the invention. Thislimited image data can then at the end of the limited scan for thesingle object, be treated in the associated computer facility during thescan of the next single object. In this way the computation of imageparameters for each individual object can be conducting during thecourse of the scan of the regions or areas of interest in the totalimage field, with, in general, the computation for one object occurringwhile the data for the next is collected.

It is also possible to decode the X-ray and BSE bit pattern to define,as a specified number, the specific mineral, phase or composition at agiven raster dwell point, while the data for the next dwell point isbeing collected. In general, since data transmission to the controlcomputer occurs only when there is a change of composition, there issufficient time for such decoding.

Acquisition of X-ray signal data may be controlled subject to BSE signalvalue or values. Any or all of the BSE value ranges (8 in the example ofFIG. 4) can be set to allow (or disallow) the collection of X-rays, withthe obviation of consequent longer dwell times necessary for "yes-no"X-ray identification for the elements present. For example, if leadsulphide or some substance having a high backscatter electroncoefficient is present, the BSE value alone can identify this substanceand X-rays need not be collected. Similarly, if an X-ray identificationof dense minerals, with high electron backscatter coefficients, but notof silicates or other gangue minerals, with lower BSE coefficients, isrequired, X-rays are taken when BSE values are above a given level only.This technique allows a considerable saving in time when only specificphases or groups of phases need to be identified by X-rays. Thetechnique also includes thresholding, as used in prior art, incombination with keeping dwell times short when the electron beam is onbackground, i.e. plastic mount material or low density mountingmaterials such as carbon or beryllium. In the present invention, fastand slow scanning (short and long dwell times) is not confined tothresholding, but can be applied to any grey level band or combinationof bands.

The real time formed digital word or mask containing X-ray 0,1 valuesfor elemental identification together with a digitized value of BSErange or ranges as generated by the system shown in FIG. 4 may bemodified by suppressing defined groups of X-ray values or groups ofvalues. For example, some or all X-ray elemental identificationsassociated with sulphides can be suppressed when the BSE signalbrightness indicates silicates. Thus at silicate-sulphide phaseboundaries for example, where X-rays due to both species are generated,creating a complex digital mask pattern, the complexity can be greatlyreduced by suppression of one group or the other, depending on BSEbrightness value.

This suppression can be made subject to analogue BSE signals, with adigital gate, reed relay or other device suppressing transmission of thespecified X-ray bits, or to the digitized value or value range of theBSE signal. In this latter case digital logic units or software can beused to suppress unwanted X-ray bits.

This procedure drastically reduces the complexity of interpretingdigital mask patterns, especially those arising at phase boundaries orother regions of ambiguity, and typically any or all of 12 elements canbe recognized simultaneously, corresponding for example to up to 30defined minerals. Thus, prima facie a matrix of 2^(n), where n is thenumber of elements, has to be decoded. By use of the present method, theproblem is typically reduced to the decoding, for n=12, or two matriceseach of only 2⁶, a total decrease in magnitude of 2⁵.

Table 1 previously referred to is an example of a set of mineral phaseswhich can be simultaneously identified during modal or image analysis ofa sample from a typical complex ore, in which at least 20 differentminerals were present. Such an analysis can be facilitated by using athreshold value and two brightness ranges for BSE signals, together,say, with 12 X-ray channels. A modification for this purpose to thesystem of FIG. 4 is shown in FIG. 5. Here, the latches 86a, 88a andcomparator 90a take the place of the latches 86, 88 and comparator 90 ofFIG. 4. The input of X-ray and BSE data is effected in a slightlydifferent fashion, however. Thus, in FIG. 5 8 BSE grey levels, observedat a particular point in the scanned raster, are made available to two8-input AND-gates 140, 142. By means of switches S.1 any or all of thegrey levels may be made to contribute to the decision whether theoutputs of gates 140 and 142 are set. A 20 bit X-ray pattern, derivedfrom count and/or X-ray ratio thresholds or windows as previouslydescribed is accepted, through gates C4, as new data only if the correctgrey level pattern is present at the inputs to gate 140. The output ofgate 142 is inverted by an inverter 144 before entry into the X-ray bitselection gates C4 and therefore X-ray data is accepted as new data onlyif a particular grey level pattern into B is not present. Switches S.2determine whether an X-ray bit is to be made subject to any grey levelcondition being present or not present. Once accepted as new data thetotal pattern in the `new data` latch 86a is compared with the data heldin latch 88a which was valid at the previous dwell point in the scannedraster and, if different, causes a change in `Status` which indicates tothe computer the need to read the pattern with its X and Y coordinates.

The total bit pattern, derived from X-rays and BSE signal conditions, isalso passed on to a cross-bar arrangement consisting of a matrix ofgates H1 . . . H28 . . . . In this cross-bar each bit of the bit patternis available to all other bits as a conditional inhibit level. It isthereby possible to remove a particular bit from the edited outputpattern right side of diagram, if a selected conditional pattern isformed by the remaining bits. The information which determines whatspecific pattern will disable a given bit is entered into controllatches G1 . . . G28 from information stored in the control computerlibrary.

X-ray generation by an electron beam occurs in a region up to a fewmicrons in diameter. Thus, within this distance from phase boundaries,X-rays from each phase are obtained. Use of BSE brightness differencesbetween different phases allows the phase boundary to be located withthe resolution of BSE SIGNALS, typically 0.1 microns, while selectedX-rays, or groups of X-rays, corresponding to the BSE values on eitherside of the boundary are used to define the compositions. X-rayscorresponding to the alternate BSE value are ingored, and the advantagesof limited data decoding are thus obtained.

This procedure also improves drastically the definition of phase orparticle boundary location in plastic or epoxy mounts where the phasematerial dips obliquely below the plastic surface. BSE signals definethe plastic or epoxy, even though X-rays are being generated by moredeeply penetrating electrons and would otherwise falsely indicate thelocation of a boundary on the true plane of polish or sectioning.

Previous reference has been made to the real time formation of X-raycount ratios which enable short-dwell times and low total number ofaccumulated counts to discriminate phases containing common elements,e.g., FeS, and FeS₂, or CuFeS₂ and Cu₅ FeS₄. The result of the ratiocomputation is a digital 1 or 0 corresponding to a ratio value fallingwithin, or not within, a defined range of values, or above or below agiven value. This result is recorded as a 1 or 0 bit in the digital maskdefining the species composition at a given raster point. Theoccurrences of such bits, and their correspondence with defined regionsof the image, can be displayed on a CRT or storage oscilloscope screenfor operator verification, and to allow adjustment of the correct ratiosettings.

The level of confidence obtained using this invention is unexpectedlymuch higher than is obtainable by simple thresholding of a series ofacceptance levels for a given X-ray peak. For example, in distinguishingthe phases FeS and FeS₂, a common and difficult problem in quantifyingmineral species, a 99% correct recognition of either phase was obtainedwith a total number of counts for each of Fe and S of only 100 to 200 ateach raster point. At these counts values, with the random statisticsgoverning X-ray production, simple windowing of count values defined ina given case as N, is subject to a standard deviation of √N or avariation of N, and confidence levels for discrimination by thresholdingon a given peak were considerably lower than those obtained from ratiovalues.

This ratio procedure also allows the discrimination of significantamounts of one element in the presence of interfering X-ray peaks ofanother. For example, the Kα X-ray peak of cobalt coincides with the Kβpeak of iron, and the presence of moderate amounts of cobalt is normallydifficult to discriminate against the iron Kβ value when low totalcounts and simple windowing are used. By taking the ratio of the countsin cobalt Kα peak position, with or without correction for background,against the counts in the iron Kβ position, the occurrence ofsignificant cobalt in the mineral or phase, even in the presence of someiron, can be readily distinguished in real time, i.e. during the imagescan, and with short dwell times. With the EDS system used in theexample, dwell times of 20 to 30 msec were sufficient. Fordiscrimination of iron Kβ from cobalt Kα, especially in the presence ofunknown amounts of iron in a cobalt phase, or for overlapping X-rayspectral lines generally, it is generally necessary to accumulatesufficient counts for spectral peak stripping or for peak heightcomparisons using software mathematical techniques and requiring timesof the order of 1 second, i.e. 1 to 2 orders of magnitude greater thanare necessary with the present invention.

In FIG. 6, single channel analysers SCA-1, SCA-2 . . . SCA-N selectX-ray or EDS pulses corresponding in energy to a particular respectiveelement. The pulses from analysers SCA-1 . . . SCA-N are counted inassociated counters CTR-1 . . . CTR-N during the alloted dwell time. Thecount from counter CTR-1 is entered, as a continually changing number N,into a digital-analogue converter 150. Digital-analogue converter outputis a voltage level rising with an increase in counts. This voltage issupplied to an analogue voltage divider 152 as input `A`. A second input`B` to divider 152 is a similarly derived voltage, the level of which isproportional to the count in one other particular counter CTR-2 . . .CTR-N and corresponding SCA output, chosen by a multiplexed arrangement,as the denominator input M to a second digital-analogue converter 154.

The divider output A/B is a voltage level which is applied to severalcomparators 156 arranged in pairs. One comparator 156 of each pair is a`lower level` comparator and the other is an `upper level` comparator.The outputs of each such pair are connected to a respective AND gate158. Lower level comparators 156 set an output bit when the dividervoltage, i.e. the ratio N/M, is greater than the level set on a 10-turndial, the upper level comparators set when the ratio is less than thatset on a second 10-turn dial.

Combinations of the described pairs of comparators 156 provide windowswhich set output bits via latches such as the latch 160 shown when aratio falls within one of these windows. To avoid acceptance of bits setwhen the denominator is zero, the computer bits are made subject to asufficient count being present in both numerator and denominator.

It is possible to use fewer, or even only one, digital to analogueconverters prior to the ratio comparison and to multiplex in, by rapidswitching of counter addresses, a sequence of pairs of numbers to beratioed. Such procedures can still be accomplished within times shortcompared with X-ray dwell times, e.g. 10 microseconds per number pair,as compared with 10-40 milliseconds for X-ray dwell times, and thus arestill performable in real time, during the progress of an image scan.Alternatively such comparisons can be made on temporarily stored spectraor spectral regions, as either at the end of each dwell period or duringthe dwell period during which data for the next raster point is beingcollected.

Quantitative mapping of three-dimensional particles or fragments,especially those containing multiple phases which must be discriminated,cannot normally be performed. Portions of a sloping or rough surfaceremain shadowed with respect to a single EDS or other X-ray detector 24,and the use of more than one X-ray detector 24 or BSE detector 26 toobtain views of different faces or shapes of the rough surface orparticle is included in the invention.

Typically, spheres of a given material such as polystyrene, glass orsteel viewed with an overhead detector show shadowed rims in theresulting image, and the BSE signals in particular collected over theupper hemisphere of such objects are not uniform.

It is therefore preferred to use an array of BSE detectors such that thecombined signals from the set of detectors gives a closely uniformresponse (with 1% to 2% variation for example) over the whole of theupper hemisphere of a given spherical object. The results in an exampleare equivalent for objects in the size range of microns to a fewmillimeters. The image of such an object can be made to appear as acompletely uniform while or grey disk against a background of darkergrey or black. For non-spherical or rougher surfaces the degree ofvariation in the BSE signal as a result of such topography for any givenspecies or phase composition is greatly reduced, and BSE discriminationof different phases on particles, fragments or rough surfaces can stillbe made.

FIG. 7 shows such an array consisting of eight Schottky Diodephotodetectors 170 (of which only six are visible) of 1 cm² active area,mounted in pairs at 45° and 671/2° to the horizontal, one pair in eachhorizontal quadrant. Thus, the photodetectors are carried by across-shaped metal bracket 172 apertured to receive the photodetectorstherebehind. The photodetectors are insulated from the frame by micawashers 174. A separate amplifier (not shown) is connected to eachdetector, and after gain adjustment, the outputs from the eightamplifiers are summed. The output each amplifies can also be separatelyconverted to a digital value and this transmitted to the controlcomputer. By extension of the procedures described in J. Lebiedzik etal. Scanning Electron Microscopy/1979 II pp. 61-66, "Use ofmicrotopography in the SEM for analysing fracture surfaces", an estimateof the slope of any portion of the surface may then be found, ratherthan the slope in one direction only. Normally, the detector array ischaracterized by an approximately axially symmetrical disposition of thedetectors, and the placing of each detector so that its face isapproximately normal to the point of impingement of the electron beam atthe horizontal plane representing the specimen surface.

The techniques of the invention permit obtaining an integrated estimateof the total concentrations of all elements present in the set ofparticles, ore section, or any other materials comprising the specimen.This is accomplished by opening the input to a multi-channel analyser,analogue-digital converter coupled with addressable memory, or othersuch device or arrangement, only during the periods when X-rays arebeing collected for identification of the composition of appropriateraster points in the sampled image field. In this way an X-ray energyspectrum is built up which represents the sum of the elementalconcentrations at the points in the image or sample field which haveactually been sampled.

This spectrum can then be deconvoluted by known or available peakstripping procedures to provide an overall elemental composition. Suchinformation is of value in its own right, but is especially andadditionally valuable in checking and normalizing the elementalcomposition of the sample as inferred from the proportions of thevarious minerals or phases identified by the image analysis procedurescomplementary to the image acquisition procedures described herein. Suchinferred values depend on assumed or known compositions and densitiesfor all of the minerals present or identified, and are often subject tosystematic errors.

                                      TABLE I                                     __________________________________________________________________________    EXAMPLE OF A COMPLEX PHASE ASSEMBLAGE IN WHICH ALL PHASES ARE                 RECOGNIZED DURING A SINGLE SCAN                                                                    EDS X-ray peak centre.sup.b , KeV                        Mineral    Nominal   1.74                                                                              2.02                                                                             2.31                                                                             2.99                                                                             3.31                                                                             3.69  4.51                                                                             5.90                                                                             6.40                                                                             8.04                                                                             8.63                                                                             10.55               No..sup.a                                                                        Phase   Composition                                                                             Element Detected                                         __________________________________________________________________________    1  Galena  Pbs       --  -- S  -- -- --    -- -- -- -- -- Pb(Lα)        2  Sphalerite                                                                            ZnS       --  -- S  -- -- --    -- -- -- -- Zn --                  3  Pyrite  FeS.sub.2 --  -- S  -- -- --    -- -- Fe -- -- --                  4  Pyrrhotite                                                                            FeS       --  -- S  -- -- --    -- -- Fe -- -- --                  5  Chalcopyrite                                                                          CuFeS.sub.2                                                                             --  -- S  -- -- --    -- -- Fe Cu -- --                  6  Tetrahedrite                                                                          Cu.sub.12 Sb.sub.4 S.sub.13                                                             --  -- S  -- -- Sb(Lα,β)                                                                 -- -- -- Cu -- --                  7  Freibergite                                                                           Cu.sub.8 Ag.sub.4 Sb.sub.4 S.sub.13                                                     --  -- S  Ag -- Sb(Lα,β)                                                                 -- -- -- Cu -- --                                                 (Lα)                                     8  Bournonite                                                                            CuSbPbS.sub.3                                                                           --  -- S  -- -- Sb(Lα,β)                                                                 -- -- -- Cu -- --                  9  Arsenopyrite                                                                          FeAsS     --  -- S  -- -- --    -- -- Fe -- -- As(Kα)        10 Hematite;                                                                             Fe.sub.2 O.sub.3 ;Fe.sub.3 O.sub.4                                                      --  -- -- -- -- --    -- -- Fe -- -- --                     Magnetite                                                                  11 Quartz  SiO.sub.2 Si(1).sup.d                                                                       -- -- -- -- --    -- -- -- -- -- --                  12 Albite  NaAlSi.sub.3 O.sub.8                                                                    Si(2).sup.d                                                                       -- -- -- -- --    -- -- -- -- -- --                  13 Calcite CaF.sub.2 --  -- -- -- -- Ca    -- -- -- -- -- --                  14 Manganiferous                                                                         (Ca,Mn,Fe)CO.sub.3                                                                      --  -- -- -- -- Ca    -- Mn Fe -- -- --                  15 Rutile  TiO.sub.2 --  -- -- -- -- --    Ti -- -- -- -- --                  16 Ilmenite                                                                              FeTiO.sub.3                                                                             --  -- -- -- -- --    Ti -- Fe -- -- --                  17 Barytes BaSO.sub.4                                                                              --  -- S  -- -- --    Ba -- -- -- -- --                                                             (Lα)                         18 Muscovite                                                                             Kal.sub.3 Si.sub.3 O.sub.10 (OH).sub.2                                                  Si  -- -- -- K  --    -- -- -- -- -- --                  19 Chlorite                                                                              (Mg,Al,Fe).sub.3 (SiAl).sub.2                                                           Si  -- -- -- -- --    -- -- Fe -- -- --                             O.sub.5 (OH).sub.4                                                 20 Sphene  CaSiTiO.sub.4                                                                           Si  -- -- -- -- Ca    Ti -- -- -- -- --                  21 Apatite Ca.sub.5 (PO.sub.4).sub.3 F                                                             --  P  -- -- -- Ca    -- -- -- -- -- --                  __________________________________________________________________________     .sup.a Phases 1 to 10 have BSE coefficients greater than 0.15, and are        grouped on that basis                                                         .sup.b All Xray peaks are Kα lines unless otherwise indicated. A        band approximately 0.2 KeV wide is centered at each peak position for         acceptance of Xrays in the peak.                                              .sup.c Pyrite and pyrrhotite are distinguished by S/Fe count                  .sup.d Two independent present levels discriminate the amount of Si in        mineral 11, Si(1), and mineral 12, Si(2) or the method of Section 9 are       used.                                                                    

                                      TABLE 2                                     __________________________________________________________________________    POLISHED SECTION EDS SPECTRA NORMALIZED IN TWO PARTS                                    Region A:                                                                           0.8- 4.2 KeV,                                                                        333 counts                                                       Region B:                                                                           4.2- 20.0 KeV,                                                                       500 counts                                                                               Sulphur/                                               Element                                                                             Count            Metal                                                  or    Peak     Time                                                                              3 E.S.d                                                                           count                                       Phase  Tilt                                                                              Region                                                                              + BGD                                                                              BGD msecs                                                                             limits                                                                            ratio*                                      __________________________________________________________________________    FeS.sub.2                                                                             0°                                                                        S     227  13  15  181/24                                                                            1.07                                        pyrite     Fe    212  19  18  170/32                                                 35°                                                                        S     231  9   40  185/18                                                                            1.04                                                   Fe    222  11  34  177/21                                                 37°                                                                        S     225  10  141 180/19                                                                            1.05                                                   Fe    214  12  42  171/23                                          Fe.sub.x S                                                                            0°                                                                        S     205  15  17  163/27                                                                            0.91                                        pyrrhotite Fe    225  12  16  180/23                                                 30°                                                                        S     211  13  33  167/24                                                                            0.89                                                   Fe    237  11  24  191/21                                                 37°                                                                        S     212  12  40  168/23                                                                            0.87                                                   Fe    243  11  26  197/21                                          Sphalerite                                                                            0°                                                                        S     169  15  15  130/27                                                                            0.99                                                   Zn    171  12  18  132/23                                                     Fe    57   15  18  34/27                                                  35°                                                                        S     177  22  27  137/36                                                                            1.00                                                   Zn    177  9   21  137/18                                                     Fe    47   13  21  26/24                                           Galena  0°                                                                        Pb    56   14  15  34/25                                                                     12(A)                                                      35°                                                                        Pb    63   14  18  39/25                                                                     18(A)                                               Chalcopyrite                                                                          0°                                                                        S     202  15  14  160/27                                                                            2.04                                        CuFeS.sub.2                                                                              Cu    99   11  16  69/21                                                      Fe    150  15  16  113/27                                                 35°                                                                        S     204  14  39  162/25                                                                            2.02                                                   Cu    101  8   26  71/17                                                      Fe    153  11  26  116/21                                          Bornite                                                                               0°                                                                        S     176  19  17  136/32                                                                            1.00                                        Cu.sub.5 FeS.sub.4                                                                       Cu    176  9   16  136/18                                                     Fe    65   15  16  41/27                                                  35°                                                                        S     170  20  47  131/33                                                                            0.91                                                   Cu    187  9   24  146/18                                                     Fe    63   12  24  39/23                                           .sup.# Covellite                                                                      0°                                                                        S     186  16  16      ˜1.0                                  CuS        Cu    190  12  16                                                  Chalcocite                                                                            0°                                                                        S     145  20  17  108/33                                                                            0.65                                        Cu.sub.2 S Cu    223  10  15  175/19                                                 35°                                                                        S     144  21  34  108/34                                                                            0.62                                                   Cu    233  9   15  187/18                                          Biotite                                                                               0°                                                                        Si    135  14  16  100/25                                                                            0.99                                                   K     71   13  16  46/24                                                      Fe    136  20  46                                                         35°                                                                        Si    147  13  39  110/24                                                                            1.07                                                   K     94   15  39  65/27                                                      Fe    138  18  74  102/31                                          Cassiterite                                                                           0°                                                                        Sn    251  27  8   204/43                                                     (.tbd.K)                                                                            128  14  8   94/25                                                      (.tbd.Ca)                                                                           129  14  8   95/25                                                      Region B       25(B)                                                      35°                                                                        Sn    267  21  13  218/34                                                     Region B       34(B)                                               Rhodonite                                                                             0°                                                                        Si    173  17  19  134/29                                                                            0.89                                                   Ca    55   15  19  33/27                                                      Mn    195  14  23  153/25                                                     Fe + Mnβ                                                                       85   12  23  57/23                                                  35°                                                                        Si    148  16  36  111/28                                                                            0.72                                                   Ca    75   18  36  49/31                                                      Mn    205  12  26  162/23                                                     Fe + Mnβ                                                                       88   11  26  60/21                                           Hematite                                                                              0°                                                                        Fe    281  9   15  231/18                                                     Region A       44(A)                                                      35°                                                                        Fe    288  8   16  237/17                                                     Region A       73(A)                                               Freibergite                                                                           0°                                                                        S     108  20  10  76/33                                                                             1.4                                                    Ag    76   15  10  50/27                                                      Sb    50   15  10  29/27                                                      Region B       20(B)                                                      35°                                                                        S     88   20  16  60/33                                                      Ag    88   15  16  60/27                                                      Sb    54   15  16  32/27                                                      Region B       26(B)                                               AlPO.sub.4                                                                            0°                                                                        Al    126  12  13  92/23                                                                             0.89                                                   P     141  12  13  105/23                                                     Region B       83(B)                                                      35°                                                                        Al    120  13  28  87/24                                                                             0.86                                                   P     142  13  28  106/24                                                     Region B       92(B)                                               Quartz  0°                                                                        Si    261  9   10  213/18                                                     Region B       44(B)                                                      35°                                                                        Si    261  8   45  213/17                                                     Region B       99(B)                                               Dolomite                                                                              0°                                                                        Ca    203  20  10  161/33                                                     Region B       39(B)                                                      37°                                                                        Ca    230  19  45  185/32                                                     Region B       56(B)                                               __________________________________________________________________________     .sup.# Count values for covellite are                                         *or count ratio for dominant two elements.                               

                  TABLE 3                                                         ______________________________________                                                           FeS.sub.2                                                                             FeS                                                ______________________________________                                        S peak               227       205                                            Remainder of region A                                                                              106       126                                            Ratio A              2.14      1.63                                           Fe peak              212       225                                            Remainder of region B                                                                              288       275                                            Ratio B              0.74      0.82                                           Ratio A/Ratio B      2.91      1.99                                           Direct S/Fe ratio    1.07      0.91                                           ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        EDS SPECTRA FROM A PYRRHOTITE PARTICLE,                                       NORMALIZED IN TWO PARTS                                                       Region A:    0.8-4.2 KeV 333 counts                                           Region B:    4.2-20.0 KeV                                                                              500 counts                                           Sample               Counts        Sulphur/metal                              and          Ele-    Peak &  Time  count                                      position     ment    BGD     msecs ratio                                      ______________________________________                                               Polished  S       205   17    0.91                                            Section   Fe      225                                                  Particle                                                                             Sloping to                                                                              S       204   16    0.90                                            detector  Fe      226                                                         Top       S       197   38    0.87                                                      Fe      226                                                         Top       S       184   44    0.92                                                      Fe      201                                                         Far right S       106   63    0.89                                                      Fe      119                                                         Far left  S        85   144   0.81                                                      Fe       95                                                  ______________________________________                                    

                                      TABLE 5                                     __________________________________________________________________________    LINE-SCAN MODAL ANALYSIS, POLISHED LEAD-ZINC ORE                              Single frame 1.2 × 0.9 mm, 25 lines, 35 microns apart, 1.2 micron       point separation,                                                             29 mm total traverse.                                                                                                        Total                                      Freiber-            Potassium Uniden-                                                                            non                            Pbs      ZnS                                                                              gite.sup.a                                                                         FeS                                                                              FeS.sub.2                                                                        Quartz                                                                            Dolomite                                                                           mica  Apatite                                                                           tified                                                                             Sulphides                      __________________________________________________________________________    VOLUME PERCENT                                                                BSE only                                                                            25.6                                                                             60.4                                                                             2.1  6.9.sup.b                                                                         --.sup.c                                                                        --  --   --    --  --   5.0                            X-ray +                                                                             25.1                                                                             60.4                                                                             1.5  6.6                                                                              0.3                                                                              0.0  0.07                                                                               3.7  0.03                                                                              1.4  5.2                            BSE                                                                           MEAN INTERCEPT LENGTH, microns                                                BSE only                                                                            24.0                                                                             43.7                                                                             10.6 28.5                                                                             -- --  --   --    --  --   9.6                            X-ray +                                                                             22.3                                                                             42.3                                                                             13.2 24.6                                                                             3.4                                                                              0.0 5.2  10.2   4.6                                                                              4.8  --                             BSE                                                                           __________________________________________________________________________     .sup.a Cu.sub.6 Ag.sub.4 (Zn,Fe).sub.2 Sb.sub.4 S.sub. 13?                    .sup.b FeS and FeS.sub.2 combined                                             .sub.c Not determined                                                    

We claim:
 1. A method of analysis in which a beam of energy is caused tofall on a spot on the surface of a sample to be analysed and X-rays thengenerated at the spot are detected by one or more detectors to producefirst signals representative of the energies of detected X-rays;comprising making a first count of the number of said first signals eachrepresentative of an energy within a relatively broad range of suchenergies and making a second count of the number of said first signalseach representative of an energy in an associated relatively narrowrange of energies about one particular energy wherein informationrelating to the relative proportion of a particular chemical element,characterized by production of X-rays of said particular energy, isobtained in the form of a normalized ratio of said second count to saidfirst count, said normalized ratio being represented by the value ofsaid second count when said first count reaches a predetermined value.2. A method as claimed in claim 1 in which an additional second count ismade, each said second count being a count of the number of said firstsignals representing the energy in a respective separate relativelynarrow range of energies about said particular energy, the particularenergy for each second count being different, whereby the assumed valuefor each second count represents a separate normalized ratio.
 3. Amethod as claimed in claim 2 wherein said first count includes at leastone of said second counts.
 4. A method as claimed in claim 2 whereinsaid first count is exclusive of said second counts.
 5. A method asclaimed in claim 3 wherein at least one said relatively narrow range ofenergies falls within said relatively broad range of energies wherebythe respective assumed value of said second count for said first signalsfalling within that relatively narrow range of energies constitutes anormalized ratio of peak energy to total energy for the spectrum ofenergies represented by said relatively broad range.
 6. A method asclaimed in claim 3 wherein an additional first count is also made, saidrelatively broad range of energies for each of said first counts beingdifferent.
 7. A method as claimed in claim 6 wherein the particularenergy or associated relatively narrow range of energies for which saidfirst signals are counted to make said second count is arranged to fallwithin the relatively broad range of energies for which said firstsignals are counted to form a respective first count, and the particularenergy or associated relatively narrow range of energies for which saidfirst signals are counted to make said additional second count isarranged to fall within the relatively broad range of energies for whichsaid first signals are counted to form said additional first count.
 8. Amethod as claimed in claim 7 wherein the said assumed value for saidsecond count and for said additional second count are those reached atdifferent predetermined values for each second count.
 9. A method asclaimed in claim 8 wherein said relatively broad ranges of energies atleast substantially co-join to form an extended spectrum of suchenergies.
 10. A method as claimed in claim 9 wherein there are two ofsaid relatively broad ranges of energies with one extending fromsubstantially 0.8 to 4.2 KeV and the other extending from substantially4.2 KeV to 20 KeV.
 11. A method as claimed in claim 2 wherein at leastone ratio of said second counts is also generated.
 12. A method asclaimed in claim 1 including detecting backscattered electrons generatedat said spot by action of said beam thereon and generating a secondsignal representative of the intensity of said backscattered electrons.13. A method as claimed in claim 12 wherein said second signal isgenerated by integrating an output signal from a detector of saidbackscattered electrons, said beam being caused to fall on said spot fora predetermined time before said integrating.
 14. A method as claimed inclaim 1 including the step of constructing an information signal fromsaid second count, said information signal containing information as tothe analysis of said sample.
 15. A method as claimed in claim 14 whereinsaid information signal is in the form of a digital word.
 16. A methodas claimed in claim 14 wherein said beam is caused to fall successivelyon numerous spots defining an image raster over the surface of saidsample, for generating a plurality of information signals, one for atleast each selected one of a set of selected spots, with at least thefirst information signal being stored.
 17. A method as claimed in claim16 wherein, for two successive said spots, corresponding informationsignals are compared, the first of these information signals beingstored and the second of these information signals being stored at leastin full form only if it differs from the first.
 18. A method as claimedin claim 16 wherein second signals generated for first and secondsuccessive spots are compared and the information signal correspondingto the second of such successive spots is abridged by not generating oronly partly generating second counts for that spot if the second signalsfor the first and second successive spots are substantially the samerelative to the other.
 19. A method as claimed in claim 18 whereincorresponding second counts for numerous spots are accumulated to givean average compositional indication for said sample over an area thereofrepresented by said spots.
 20. A method as claimed in claim 18 wherein afirst set of said information signals is first generated and from thisfirst set are generated maximum and minimum orthogonal co-ordinates ofan area in said image raster for which the information signals of saidfirst set have a particular characteristic, a further set of saidinformation signals then being generated from said area of said rasterdefined by said maximum and minimum orthogonal co-ordinates. 21.Apparatus for material analysis including:energy generating anddirecting means for causing a beam of energy to fall on a spot on thesurface of a sample of the material to be analysed; detector means fordetecting X-rays generated at said spot and for producing first signalsrepresentative of the energies of detected X-rays; first accumulatingmeans coupled to accumulate a first count of the number of said firstsignals each representative of an energy within a first relatively broadrange of energies; second accumulating means for accumulating a secondcount of the number of said first signals each representative of of anenergy in an associated relatively narrow range of energies about oneparticular energy; presettable means coupled to said first accumulatingmeans and responsive, on said first count reaching a predeterminedvalue, to control said second accumulating means to hold the value ofsaid second count then assumed, whereby said assumed value represents anormalized ratio of said second count to said first count whichnormalized ratio is dependent on the proportion of a particular chemicalelement in said sample.
 22. Apparatus as claimed in claim 21 includingat least one further second accumulating means, each second accumulatingmeans being coupled to said detector means for providing a respectivesaid second count; each second count being a count of the numbers ofsaid first signals representing the energy in a respective separaterelatively narrow range of energy, about said particular energy andrepresenting a respective normalized ratio.
 23. Apparatus as claimed inclaim 22 including at least one further first accumulating means, eachfirst accumulating means being coupled to said detector means forproviding a respective first count, the first counts being counts of thenumbers of said first signals each representative of the energy within adifferent respective relatively broad range of such energy. 24.Apparatus as claimed in claim 23 arranged whereby, for each relativelybroad range of energies for which a first count is accumulated in arespective first accumulating means, there is therewithin at least oneparticular energy or associated relatively narrow range of energies forwhich each second count is accumulated in a respective secondaccumulating means.
 25. Apparatus as claimed in claim 24 wherein saidpresettable means is arranged whereby the predetermined value for atleast one said first count is different from the predetermined value forat least one other first count.
 26. Apparatus as claimed in claim 23wherein each first accumulating means comprises a first discriminatordevice coupled to receive said first signals and operable to produce afirst output signal when a first signal is representative of an energywithin a respective relatively broad range of energies, and a firstcounter coupled to said first discriminator whereby a number of saidfirst output signals is accumulated in said counter to provide saidfirst count; each second accumulating means including a respective firstdiscriminating device coupled to receive said first signal operable toproduce a second output signal when a said first signal isrepresentative of an energy within a respective relatively narrow rangeof energies and a second counter connected to the respective seconddiscriminator means for accumulating a number of second output signalsreceived thereby to provide a respective second count; said presettablemeans comprising a third counter and a comparator, said comparator beingconnected to compare a count in use held in said third counter andrepresenting said predetermined value with the count in said firstcounter and operable on coincidence of the counts in the third and firstcounters to generate a hold signal, said comparator being connected toat least one associated second counter for latching the count held ineach associated second counter at said assumed value when said holdsignal is generated.
 27. Apparatus as claimed in claim 26 wherein thereare two of said first accumulating means, each arranged for applying arespective hold signal to at least one respective associated secondcounter and there being a separate presettable means for each firstaccumulating means.
 28. Apparatus as claimed in claim 27 wherein gatemeans is provided coupling to the third counter of the presettable meansassociated with one of said first accumulating means and selectivelyoperable to decrement the count in said third counter from an initiallypreset value in accordance with the number of said second output signalsreceived from at least one of said discriminator means.
 29. Apparatus asclaimed in claim 28 including divisor means coupled to at least two ofsaid second counters and operable to produce a ratio signalrepresentative of the ratio of the assumed value of said second countsin those second counters.
 30. Apparatus as claimed in claim 29 includinga backscattered electron detector means for detecting backscatteredelectrons generated at the said spot pursuant to incidence of said beamthereon and operable in use to generate a second signal representativeof intensity of such backscattered electrons.
 31. Apparatus as claimedin claim 30 wherein said backscattered electron detector means includesmeans operable to generate said second signal by integrating an outputfrom a backscattered electron detector device forming part of saidbackscattered electron detector means and delay means responsive toinitiation of incidence of said beam on said spot to delay the beginningof said integration for a predetermined period.
 32. Apparatus as claimedin claim 31 including means for generating an information signalrepresentative of analysis of said sample at said spot.
 33. Apparatus asclaimed in claim 32 wherein said means for constructing an informationsignal includes digital means producing said information signal as adigital word, individual bits of which word are representative of atleast selected ones of said second signal magnitude, said word includingindividual bits and collectively representing at least one of theintensity of said second signal and of one of said assumed values ofsaid second count of at least one of said ratios of said second counts.34. Apparatus as claimed in claim 33 including means for directing saidbeam in a raster pattern on said sample said raster pattern being madeup of an array of spots and said apparatus being operable to generate aseparate information signal at each spot in said raster.
 35. Apparatusas claimed in claim 34 including means for storing information signalsfor each said spot.
 36. Apparatus as claimed in claim 35 includingcomparator means operable to compare the second signal generated for twosuccessive spots in said raster pattern and operable to inhibitgeneration of at least one component of the information signal for thesecond occurring of these successive spots or at least to inhibitstorage thereof on detection of a condition of substantial identitybetween the second signals for the two successive spots.
 37. Apparatusas claimed in claim 35 including comparator means for comparing twosuccessively generated information signals and for suppressing storageof one thereof on detection of a condition of substantial identitytherebetween.
 38. Apparatus as claimed in claim 34 including means foraccumulating second counts and/or said second signals for numerous saidspots to provide accumulation signals representative of average analysisof said sample over said raster or part thereof.
 39. Apparatus asclaimed in claim 34 operable to generate a first set of said informationsignals by scanning said raster and to generate therefrom maximum andminimum orthogonal coordinate signals representing an area in saidraster for which the information signals of said first set have aparticular characteristic and operable to generate a further set of saidinformation signals from said area.
 40. Apparatus as claimed in claim 30wherein said backscattered electron detector means includes an array ofbackscattered electron detectors together with signal combining meansfor combining signals generated by the detectors.
 41. Apparatus asclaimed in claim 40 wherein said detectors are positioned such thatcombined signals resulting from incidence of said beam on a single saidspot are substantially independent of the slope of the surface at thatspot.
 42. A method as claimed in claim 2, wherein said first countconsists of the sum of all or of any one or more of the second counts.